Observation of cosmic ray modulation and possible detection ofthe solar flares with GRAPES III muon telescopes at Ooty.
S.Kawakami2, Y.Aikawa2, N.Ikeda2, S.K.Gupta1, Y.Hayashi2, N.Ito2, H.Kojima3, D.K.Mohanty1,K.Sasaki2, M.Sasano2, K.Sivaprasad1, B.V.Sreekantan1, H.Tanaka2, and S.C.Tonwar1
1

Tata Institute of Fundamental Research, Mumbai 400 005, INDIA

2

Faculty of Science, Osaka City University, Osaka 558 8585, JAPAN3

Nagoya Woman University, Nagoya, JAPANABSTRACT
We have observed the variation of muons (>1GeV) intensity for one year at Ooty (latitude
11.4 longitude 76.7) with the large muon detectors of GRAPES III (total area of 560 m2).
These detectors were constructed primarily to observe the muons in the Air Shower. Since ourrecording system store the muon counting rate each one second, we can get the variation ofcosmic rays for very short time interval, 1 sec onward. Since the total counting rate of muons is
so high, about 5.3 x 104/sec, even rapid change can be analyzed with reasonable statistics. We
present the variation of muons in solar flare event with 5 min. interval and for one month with
one hour interval.
Recently we introduced the angle measurement system for individual muons to one of theunit (36m2). We are planning to extend it for all the units.

INTRODUCTION
So far quite a few attempt have been made to understand the cause of sidereal time variationof cosmic rays.(D.L.Hll et. al.) But the origin of the sidereal diurnal variation are not understoodyet. So we planned to have a muon telescope with huge effective area and narrow angle
resolution to observe this sidereal variation with great accuracy. Since our telescope is locatednear equator (latitude 11.4 degree), it has an advantage of looking northern and southernhemisphere with same detector.
Using same detector we are hoping to detect the short time variation in Solar flare event at
ground level. Those rapid change would give the clue to understand the mechanisms of particle
acceleration in their earlier stage.

DETECTORS AND EXPERIMENTAL CONDITIONS
The details of the muon telescope is to be found in Proceedings of last ICRC (N.Ito et. al.).
Our muon telescope consisted of four layers of proportional counters seperated with concreteof 15 cm thick each. Total thickness of detector is 550g/cm2, so the minimum energy ofpenetrating muon is about 1 GeV. We are recording the total counts of 4-layer penetrating
muons in each second. All the observed data presented here does not have any angularinformation.
Though the daily temperature variation outside detector is around 15 degrees, it is within one
degree at detector. The atmospheric pressure changes very much periodically with 12 hours
cycle. Fig.1 shows a typical example of the variation of muon intensity and 12 hours variation                 !"                      

of atmospheric pressure for a month.

ANALYSIS OF SOLAR FLARE EVENT
Nagoya’s STE Lab group observed the low energy neutron associated with the Solar flare. If
the high energy neutrons are to be produced during the Solar flare event, our present detector
might be able to see increase at that incidence. We selected the solar flare event greater than M
5 class which occurred at around 10:00 to 14:00 local time. Observation period is between
March 1988 and January 1999. Some of the typical example is shown in fig.3 and 4. Counting
rate is summed up for 5 minutes.
We have examined each raw data (one second interval) whether there is any misbehavior due
to hard and soft ware problems and then summed up for all the units together. We tried to findout the optimum time interval to see the change in intensity. For this purpose 1 min. , 3 min and
5 min interval were checked, but none of the data set showed significant increase or change incounting rate during those solar flare time.
D e v ia tio n fro m A v e ra g e fo r S o la r F la rea t 8 :1 4 to 8 :3 2 1 8 /0 8 /1 9 9 8 U T

Deviation from Average0 .0 0 40 .0 0 30 .0 0 20 .0 0 1
0-0 .0 0 1-0 .0 0 2-0 .0 0 3-0 .0 0 4
1 1 :0 0

1 2 :0 0

1 3 :0 0

1 4 :0 0

1 5 :0 0

1 6 :0 0

L o c a l T im e ( U T + 5 .5 h )F ig u re 3 : M u o n in te n s ity d u rin g th e S o la r f la r e o n 1 8 /0 8 /1 9 9 8                                                    !  "    #  $  % & '   ( '     )* +'   %             

DISCUSSIONS AND CONCLUSIONSWe could not find any significant increase of muons associated with solar flare. As Imentioned earlier, present data is just total counting rate of penetrating muons, so the sensitivityof our detectors would be limited.
As a next step of the experiment, we introduced the angle measurement system for individualmuons. The accuracy of this angle measurement is around 15 degrees. This is due to thelimitation of size of proportional counters and their vertical separation. Minimum time
resolution of observation is one second. Installing this systems into one of the unit we started
the observation from March 1999 and we are going to install this recording systems into another10 units within this fiscal year. With this arrangement we are expecting to detect much detailed
sidereal variation in a short observation time and the rapid change of high energy neutron
intensity in Solar flare event, if any.
ACKOWLEDGMENTS
We are grateful to the Ministry of education of Japan for their partial financial support for thisexperiment. The authors wish to thanks Prof.V.S.Narasimham and other member of the TIFR-OCU
Proton Decay Collaboration for the loan of proportional counters used in the muon detector. We arealso happy to acknowledge valuable contributions of N.V.Gopalakrishnan, G.Paul Francis, A.Peter,K.C.Ravindran, B.Sreenivasa Rao, S.Thirunavukkarasu, K.Viswanathan and V.Viswanathan during
the installation, operation and maintenance of the instrumentation for the GRAPES III array. The
help and cooperation of the Radio Astronomy Center for providing site facilities for the GRAPES
III array are gratefully acknowledged

REFERENCES
N.Ito et. al.

Proced. 25th ICRC Durban HE 6.1.9

D.L.Hall et. al. J. Geo. Res. 104 A4, p6737 1999.